Highly pure S-(−)-methyl lactate constitutes an important product having interesting possibilities of application at an industrial level. It can be used in pharmaceuticals and in the production of high purity lactic acid. The lactic acid so produced can be used in the production of dairy products, as an acidulating agent in the alimentary field, as an intermediate for the production of plasticizer agents, adhesives, pharmaceutical products, in the production of lactates, as a mordant in wool dying and many others. Similarly, the high purity lactic acid prepared from high purity methyl lactate has considerable prospects of industrial development in the field of production of biocompatible and biodegradable polymers which are useful for manufacturing bags, application films, in the field of sanitary and medical applications and so on.
The manufacturing method for methyl lactates as known from prior art, in general, comprises of two steps. In step 1, alkali metal lactate is reacted with concentrated or dilute sulfuric acid. The alkali metal sulfate is produced as the side product along with the dilute aqueous solution of lactic acid. In step 2, dilute lactic acid solution is concentrated and esterified with methanol using a catalyst. The resultant mixture of methyl lactate, methanol and water is further purified to give product i.e. methyl lactate. Various methods of manufacturing methyl lactate based on the above general method have been suggested in the past as cited in the following patented literatures: Reference may be made to U.S. Pat. Nos. 2,290,926, 2,406,648, 2,390,140, 2,334,524, 2,350,370 and 2,434,300, wherein a method for manufacture of methyl lactate has been disclosed, but suffer from various drawbacks such as impure product, need for expensive and energy intensive processes for separation of product, residue of sodium sulphate, corrosiveness and several others.
References may be made to U.S. Pat. No. 6,342,626, wherein method for manufacture of methyl lactate from 73% lactic acid in two stages at high temperature 200° C. and high pressure of 20 kg/cm2 is described. The process is carried out in two stages; first stage equilibrium conversion of lactic acid reported is about 80% and after isolating byproducts of the first stage, the second stage gives remaining conversion of lactic acid. The drawbacks are the use of higher temperature and pressure and corrosive nature of lactic acid increases the cost of the commercial manufacturing setup. Further drawback is that, the lactic acids can recemize at such higher temperature (C. H. Holtan, Lactic Acid, properties and chemistry of lactic acid derivatives, Printer Oswald Schmidt KG Leipzig, 1971, page 149). Further, the optical purity of methyl lactate and the chemical purity of methyl lactate produced are not reported.
References may be made to U.S. Pat. No. 5,453,365, wherein a continuous process for the preparation of lactates by fermentation of mixtures containing sugars, conversion of the lactic acid obtained during fermentation to its salts, followed by esterification is disclosed, in which:    a) An alkaline earth metal carbonate or bicarbonate is added to the fermentation liquor so as to neutralize the resultant lactic acid to a degree of at least 90 mol %,    b) The resulting fermentation liquor is adjusted to pH 7 to 13 by the addition of NH3 and CO2 and the resultant precipitates are separated, and    c) The resulting, purified ammonium lactate solution is esterified with an alcohol.
The general method of preparation of methyl lactate reported in the prior art is by the acidification of alkali metal lactate to generate crude lactic acid and alkali metal sulfate as the byproduct, followed by esterification of crude lactic acid by co-current method of reactive distillation where, the product methyl lactate, byproduct water and excess alcohol is taken out of reactive distillation still by maintaining the still at higher temperature. This leads to the accumulation of the acidity at the reactive distillation still and gives rise to the undesired byproducts like hydroxyl methyl furfural, 2-pentene-1-ol etc. It is known that in the product mixture of alcohol, water and methyl lactate obtained from the co-current reactive distillation, methyl lactate forms an azeotrope with water, the isolation of methyl lactate in pure anhydrous form from the mixture of alcohol, water and methyl lactate is difficult. In the prior art reported, there is no mention of the impurity profile of the product and its optical purity. Further to it, the reported methods use alkali metal lactates to produce crude lactic acid and produce waste alkali metal sulfate. Further drawback of reported methods are that the crude lactic acid generated after reacting alkali metal lactate with sulfuric acid and maintained at very low pH value, the crude lactic acid being very corrosive in nature and at high temperature it is more corrosive in nature, the manufacturing setup and it's material of construction needs to be selected carefully and it is very capital intensive in nature. Further drawback of these reported methods are that the alkali metal is wasted as alkali metal sulfate and the alkali can not be recycled to the fermentation section where alkali metal lactates are produced.